1. Field of the Invention
The present invention relates to an optical coherence tomography apparatus, and in particular relates to an optical coherence tomography apparatus wherein a light beam from a light source is swept to irradiate a predetermined area of an object to be observed, and optical interference is used to detect light reflected from the object in order to obtain tomographic information of the object.
2. Description of the Prior Art
Apparatuses in which interference from a low-coherent light (partially coherent light) is used to create an image from tomographic information of an object to be observed (optical coherence tomography (OCT)) have conventionally made it possible to form and observe a desired tomographic image of the object in a contactless and noninvasive manner. Accordingly, such apparatuses are particularly useful when observing living organisms, and have started to be used in routine clinical opthalmological examinations, dermatological diagnoses, endoscopic applications and other medical operations. They have also been considered to be useful in regard to applications involving testing equipment in industrial fields.
For example, Document D1, Japanese Laid-open Patent Publication No. 1992-174345 (Japanese Examined Patent Application (Kokoku) No. 1994-35946), discloses an example of early OCT in which irradiation light is shifted in frequency to generate a reference beam, which is combined with light reflected from an object to be measured to produce a beat component. The beat component is then detected to obtain a reflected tomographic image of the object.
Document D2, Japanese Laid-open PCT Publication 1994-511312 (Patent No. 3479069), discloses an apparatus comprising an interferometer that has a light source having short coherence length characteristics and an optic fiber, phase modulation means and a lateral sweeping mechanism that are disposed on an optical path of a probe light beam toward a sample, an ultrasonic light modulation element disposed on an optical path of a reference light beam, control means for moving an optical path length in the light axis direction, and the like. In this arrangement, interference light created by the reference light beam and the probe light beam guided via the optic fiber is detected to obtain an image of a tomographic image of the sample.
Document D3, Japanese Laid-open Patent Publication No. 2000-126188 (Patent No. 3318295), discloses an optical tomography observation apparatus including a light source for generating low-coherent light and an optic fiber interferometer. The apparatus is usefully combined with a structure constituting an end part of an endoscope, body cavity scope, or other instrument via one optical path of the interferometer. Document D3 discloses a technique involving the use of the endoscope or other instrument inserted into a body cavity, wherein a CCD or other device provided as a conventional observation device is used to provide a two-dimensional reflected image of a diseased tissue and a tomographic image in the depth direction thereof that is obtained by detecting an interference signal from an interferometer.
Document D4, Japanese Laid-open Patent Publication No. 1996-206075 (Patent No. 3549961), discloses a configuration wherein a light beam generated by a light source is split into a sample beam path and a reference beam path, the light beams that returns via the paths are superimposed and guided to a detection beam path, and an interference signal obtained by a detector is processed to derive therefrom a tomographic image of a cornea in an eye of a subject. In Document D4, a helical reference mirror provided on the reference beam path is used to perform a scan in the optical axis direction, and a reflective mirror on the reference beam path is then moved to perform a depth scanning in accordance with the curve of the cornea, thereby reducing time for data collection.
Document D5, Japanese Laid-open Patent Publication No. 1998-332329 (Patent No. 3332802), discloses a configuration that employs a semiconductor laser light source with a beam frequency capable of being swept, a Michelson interferometer, and a one- or two-dimensional image-capturing device. A video signal that is output in the frequency sweeping period is subjected to Fourier transformation to produce a tomographic image. Such a system is advantageous in that a sweeping mechanism for performing a mechanical movement in the optical axis direction is not additionally required, a stable interference optical system can be established, and measurements can be obtained in a short amount of time.
Document D6. Japanese Laid-open Patent Publication No. 1999-325849, discloses a configuration wherein a light beam is split into a reference arm and a measuring arm, and a spectrograph is used to detect the intensity of light that is produced by interference of a measuring beam obtained via the measuring arm with a reference beam obtained through the reference arm. The reference arm is provided with means for changing the phase of the light, and a signal from the spectrograph is analyzed to produce an optical tomographic image of a transparent, partly transparent, or opaque object.
Document D7, Japanese Laid-open PCT Publication No. 2003-516531, discloses an optical mapping device wherein an optic fiber interferometer and a bulk interferometer are used in combination, modulating means is introduced into one optical path, means is provided for changing the length of the optical path, and tomographic information in the depth direction of an object is derived based on an output signal from the interferometers.
Document D8, Japanese Laid-open Patent Publication No. 2001-330558 (Patent No. 3594875), discloses a system wherein a light beam from a light source is split into a signal optical path that passes through an object to be measured and a reference optical path that travels via a predetermined reflecting mirror, and an interference optical system is provided with two CCD sensors for receiving a periodically isolated interference beam that is split into two. The two CCD sensors receive interference light pulses that have different phases and produce signals, which are processed to provide image information of an interior layer of the object.
Document D9, Japanese Laid-open Patent Publication No. 2005-245740, discloses an OCT apparatus with an interferometer comprising means for forming a plurality of interference images differing in phase, and means for extracting the plurality of interference images by high-speed switching. The plurality of extracted interference images are detected using CCD image sensors, and the plurality of detected images is processed to form a tomographic image.
Document D10, Japanese Laid-open Patent Publication No. 2006-116028, discloses a measuring device composed of an interferometer and a spectrometer, wherein a light beam from a light source is focused into the shape of a line on an object being measured, and the observation light from the object being measured is detected by a two-dimensional image sensor via a spectrometer. The detected signal from the image sensor is subjected to Fourier transformation and other calculation processing, whereby sectional information of the object to be measured is quickly obtained according to the rate of computation.
Document D11, Japanese Laid-open Patent Publication No. 2006-322767, discloses a configuration in which a light beam from a light source is split into a probe beam and a reference beam, and the probe beam is directed on an object to be measured via moving means used for movement in the optical axis direction and means for performing an orthogonal scanning in relation to the optical axis. Light reflected from the object to be measured mixes with the reference beam traveling via a fixed reflection surface to produce interference light, which is detected in accordance with the sweeping of the sweeping means and movement means to provide reflective intensity information of the interior of the object.
However, the scanning in the depth direction of the object to be observed in Documents D1 through D4 is performed by moving a reflective mirror for a reference beam in the optical axis direction. Therefore, the focus of the irradiation light (probe light) directed on the object to be observed cannot be optimally maintained for the entire region of the tomographic image, and complications are encountered in achieving higher levels of resolution in the in-plane direction orthogonal to the optical axis (depth direction). Additionally, in the configurations shown in the above Documents, the movement of the reflective mirror in the optical path of the reference light limits the rate at which the detection signal is processed, making it difficult to obtain the tomographic image of the object at high speed.
In contrast, Document D5 discloses an OCT system using a light source with a light wavelength swept (also referred to as “swept source method”). Problems are presented with this system in that a specialized type of laser light source that can stably control the frequency of the light over a desired range is required; a light source of this type is limited in terms of variety, wavelength range, and other aspects; and the light source itself is expensive.
Document D6 discloses an OCT system in which a spectrograph is used in the detection system (“spectral-domain method”). Advantages of this system are that the tomographic information is extracted based on numeric calculations, making mechanical scanning in the depth direction unnecessary; however, problems are presented in that the characteristics of the spectrograph limit the measurement range in the depth direction, and resolution is not readily improved in the direction orthogonal to the depth direction.
The configuration shown in Document D7 is advantageous in that a scanned reflection image and a tomographic image are simultaneously obtained. However, problems are presented in that the rate at which images are captured depends on light beam sweeping being performed over two optical axes, and the rate at which images are captured cannot readily be increased without the use of expensive modulating means or a specialized scanning system.
The configuration disclosed in Document D8 is advantageous in that tomographic images can be extracted at high speed via a simple computation process according to the video rate of a CCD camera. However, problems are presented with this system in that accurate positioning of the two CCD devices is difficult, and when a living organism or other object having a strong scatter factor is observed, the presence of strong background light superimposed as a direct-current component in the detector makes it difficult to improve the gradation of a signal component that contains tomographic information.
In the configuration shown in Document D9, advantages are presented in that a tomographic image can be obtained at high speed using a simple computation process. However, drawbacks are presented in that the light source is not used efficiently, and a high-cost, specialized switching light source or other such configuration is necessary when the system is implemented in practice. Additionally, the system in Document D9 has the same problems as in Document D8; i.e., when a living organism or other object having a strong scatter factor is observed, the presence of strong background light superimposed as a direct-current component in the detector makes it difficult to improve the gradation of a signal component that contains tomographic information.
Document D10 discloses a spectral domain system that improves the invention described in Document D6. The system in Document D10 requires no mechanical sweeping, and enables images of cross-sectional information to be obtained at high speed. However, the same problems as in Document D6 are also presented in this system, insofar as the measurement range in the depth direction is limited by the characteristics of the spectrograph, and the resolution is not readily increased in the direction orthogonal to the depth direction.
Document D11 proposes a system having good practical utility wherein the resolution is improved in the direction orthogonal to the optical axis, and improvements are also made while taking into account practical aspects such as the cost of the devices and the simplicity of adjustment in the optical system. However, in this system, increasing the speed of the modulating means is particularly difficult, and the time necessary to obtain one plane of a tomographic image cannot readily exceed, for example, two to three frames per second. Therefore, a problem is presented in applications involving fast-moving living organisms.
It is therefore an object of the invention is to provide a highly practical optical coherence tomography apparatus with a simpler and less expensive configuration that makes it possible to observe tomographic images of an object at high speed, for example, at 30 or more frames per second, while preserving high levels of resolution, gradation, and contrast.